全文获取类型
收费全文 | 48655篇 |
免费 | 8214篇 |
国内免费 | 11342篇 |
专业分类
测绘学 | 4293篇 |
大气科学 | 5336篇 |
地球物理 | 10188篇 |
地质学 | 29891篇 |
海洋学 | 5575篇 |
天文学 | 2172篇 |
综合类 | 3329篇 |
自然地理 | 7427篇 |
出版年
2024年 | 264篇 |
2023年 | 706篇 |
2022年 | 1406篇 |
2021年 | 1582篇 |
2020年 | 1701篇 |
2019年 | 1920篇 |
2018年 | 1549篇 |
2017年 | 1773篇 |
2016年 | 1960篇 |
2015年 | 2115篇 |
2014年 | 2581篇 |
2013年 | 2545篇 |
2012年 | 2860篇 |
2011年 | 3032篇 |
2010年 | 2605篇 |
2009年 | 3090篇 |
2008年 | 3089篇 |
2007年 | 3409篇 |
2006年 | 3283篇 |
2005年 | 2889篇 |
2004年 | 2715篇 |
2003年 | 2625篇 |
2002年 | 2345篇 |
2001年 | 2052篇 |
2000年 | 1914篇 |
1999年 | 1752篇 |
1998年 | 1511篇 |
1997年 | 1419篇 |
1996年 | 1288篇 |
1995年 | 1082篇 |
1994年 | 1117篇 |
1993年 | 948篇 |
1992年 | 747篇 |
1991年 | 533篇 |
1990年 | 476篇 |
1989年 | 378篇 |
1988年 | 290篇 |
1987年 | 189篇 |
1986年 | 122篇 |
1985年 | 86篇 |
1984年 | 34篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1981年 | 20篇 |
1980年 | 24篇 |
1979年 | 28篇 |
1978年 | 38篇 |
1977年 | 24篇 |
1975年 | 4篇 |
1954年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
1993/1994年南极夏季对中国南极长城湾及附近海区表层海水四种营养盐浓度及其分布状况作了调查。结果表明,NH4-N、NO3-N、NO2-N及PO4-P的大致浓度分别是2.13、7.07、0.74和1.12μg·dm-3。一般来说,湾内高于湾外氮盐于12月份常出现高值。四种形态的无机盐类之最高、最低浓度出现的时间和站位不尽一致。∑N/P递减次序大致是12月,2月,1月。水温变化与NH4-N、PO4-P浓度和∑N/P的变化常较一致,而与NO2-N则常不一,一些站位上营养盐浓度偏低可能与浮游植物摄取有关。结果显示了调研海区的浅水内湾特征。浮游植物所需营养充分。该湾仍维持正常状态。 相似文献
993.
Abstract. The distribution of Ibla cumingi DARWIN on different types of hard substrata along 30 km of coast in the Gulf of Elat was studied. I. cumingi was found among and underneath Tetraclita squamosa rufotincta , in oyster beds, underneath slabs of beachrock, and in fissures in igneous rocks. In the Tetraclita belt, I. cumingi is most frequent in empty shells of dead Tetraclita while in the oyster bed it is common inside the meshwork made by the oysters. The distribution is regarded as clustered distribution. The density and size of the clusters of I. cumingi is determined by the available sheltered space within the midlittoral zone. The form of the habitat determines the shape of the animals. Those found in the oyster bed and fissures are usually longer than those found in the Tetraclita belt due to the depth of the fissures or to the bigger spaces within the oyster bed. I. cumingi is orientated so that the capitulum with the cirri is projecting towards the open water, and the water current. It is concluded that I. cumingi can withstand the physical stress encountered in a tropical intertidal zone, but that the distribution pattern is caused by predation. 相似文献
994.
In recent years many fuller ship hull forms have been designed and constructed in various shipbuilding countries, but the data available for the development of the fuller forms are inadequate from the point of view of preliminary ship design. In this paper the authors describe how they have systematically tested vessel forms of block coefficients ranging from 0.80 to 0.90. The analysis and presentation of the test results have been made in such a manner that designers can produce hull forms like those of tankers and other bulk carriers quickly and reliably. 相似文献
995.
Summary. Due to the foreground extinction of the Milky Way, galaxies appear increasingly fainter the closer they lie to the Galactic
Equator, creating a “zone of avoidance” of about 25% in the distribution of optically visible galaxies. A “whole-sky” map
of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity
of the Local Group with respect to the Cosmic Microwave Background and velocity flow fields such as in the Great Attractor
region.
Various dynamically important structures behind the Milky Way have only recently been made “visible” through dedicated deep
surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and
X-ray) for galaxies in the Zone of Avoidance are reviewed, including a discussion on the limitations and selection effects
of these partly complementary approaches. The uncovered and suspected large-scale structures are summarized. Reconstruction
methods of the density field in the Zone of Avoidance are described and the resulting predictions compared with observational
evidence. The comparison between reconstructed density fields and the observed galaxy distribution allow derivations of the
density and biasing parameters and b.
Received 4 April 2000 / Published online 18 July 2000 相似文献
996.
Licia Verde Limin Wang Alan F. Heavens Marc Kamionkowski 《Monthly notices of the Royal Astronomical Society》2000,313(1):141-147
Cosmic microwave background and large-scale structure data will shortly improve dramatically with the Microwave Anisotropy Probe and Planck Surveyor , and the Anglo-Australian 2-Degree Field and Sloan Digital Sky Survey. It is therefore timely to ask which of the microwave background and large-scale structure will provide a better probe of primordial non-Gaussianity. In this paper we consider this question, using the bispectrum as a discriminating statistic. We consider several non-Gaussian models and find that in each case the microwave background will provide a better probe of primordial non-Gaussianity. Our results suggest that if microwave background maps appear Gaussian, then apparent deviations from Gaussian initial conditions in galaxy surveys can be attributed with confidence to the effects of biasing. We demonstrate this precisely for the spatial bispectrum induced by local non-linear biasing. 相似文献
997.
998.
We investigate the behaviour of galaxy clustering on large scales using the PSC z catalogue. In particular, we ask whether there is any evidence of large-scale fractal behaviour in this catalogue. We find the correlation dimension in this survey varies with scale, consistent with other analyses. For example, our results on small and intermediate scales are consistent those obtained from the QDOT sample, but the larger PSC z sample allows us to extend the analysis out to much larger scales. We find firm evidence that the sample becomes homogeneous at large scales; the correlation dimension of the sample is D 2 =2.992±0.003 for r >30 h −1 Mpc. This provides strong evidence in favour of a universe that obeys the cosmological principle. 相似文献
999.
Xin Wang Xuelei Chen Zheng Zheng † Fengquan Wu Pengjie Zhang Yongheng Zhao 《Monthly notices of the Royal Astronomical Society》2009,394(4):1775-1790
The Large Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) is a dedicated spectroscopic survey telescope being built in China, with an effective aperture of 4 m and equipped with 4000 fibres. Using the LAMOST telescope, one could make redshift survey of the large-scale structure (LSS). The baryon acoustic oscillation features in the LSS power spectrum provide standard rulers for measuring dark energy and other cosmological parameters. In this paper, we investigate the measurement precision achievable for a few possible surveys: (1) a magnitude-limited survey of all galaxies, (2) a survey of colour-selected luminous red galaxies (LRG) and (3) a magnitude-limited, high-density survey of z < 2 quasars. For each survey, we use the halo model to estimate the bias of the sample, and calculate the effective volume. We then use the Fisher matrix method to forecast the error on the dark energy equation of state and other cosmological parameters for different survey parameters. In a few cases, we also use the Markov Chain Monte Carlo method to make the same forecast as a comparison. The fibre time required for each of these surveys is also estimated. These results would be useful in designing the surveys for LAMOST. 相似文献
1000.